The present invention relates to a method for controlling alkaline pulping process, especially the sulphate process of pine and birch, by utilizing the relative composition of the alifatci acids dissolved into the cooking liquid.
In alkaline pulping processes the lignin which is present in the raw material of wood and bonds the fibers of cellulose together is removed under strongly alkaline circumstances, at which spliting of the polymeric carbohydrates, cellulose and hemicellulose takes place, which reduces the overall yield (Sjostrom, E., Wood Chemistry, Foundamentals and Applications, Academic Press, New York, 1981). Thus, the organic ingredients, dissolved into the waste liquor, consist of the substances extracted from wood and, in addition, of the disintegration products of lignin as well as of the aliphatic acids formed as a result of the spliting reactions taking place within the carbohydrate chains.
The fraction of alifatic acids, included in the waste liquor consists of evaporable acids (formic and acetic acids) as well as hydroxymonocarboxylic and hydroxydicarboxylic acids (Alen, R., Neimela, K. and Sjostrom, E., J. Chromatogr. 301 (1984) 273). The hydroxymonocarboxylic acids of the compounds mentioned above form the most significant fraction of acids present in the waste liquor and it has been concluded that over 20 various compounds belong to this group of acids. The total number of the acids formed in the hard wood pulping process is similar to that formed in the soft wood pulping process, but the relative proportions of the acids differ somewhat from each other. In addition, the cooking circumstances applied in each pulping process have an effect upon the acid contents in question. The formation of acids in the pulping process has been studied to some extent (Malinen, R. and Sjostrom E., Paperi Puu 57 (1975) 728, Niemela, K., Alen, R. and Sjostrom, E., Holzforschung (1985) under print), at which differences in the formation rates of the acids have been detected, as a concequence of which the acid compositions in the waste liquor will change during the progress of the delignification process.